Polybutylene terephthalate (PBT) is an important and commercially valuable engineering thermoplastic. The high crystallinity and rate of crystallization of polybutylene terephthalate make the resin quite resistant to organic solvents. However, the heat resistance of polybutylene terephthalate is not completely satisfactory for many purposes owing to its relatively low glass transition temperature (Tg) of only 52.degree. C. Additionally, polybutylene terephthalate must be processed above the melting point of the polymer (225.degree. C.). Since the polymer melt has very low viscosity and strength, unmodified polybutylene terephthalate is not well suited for certain types of forming or molding processes such as blow molding and thermoforming.
To improve the heat resistance of polybutylene terephthalate, the polyester has been blended with copolymers of styrene and maleic anhydride as described in U.S. Pat. No. 3,644,574, Japanese Kokai No. 57-67643, and European Patent Publication No. 257,693. However, it is also well known that combining these resins by melt mixing in an extruder results in molded products having a tendency to delaminate due to the gross immiscibility and incompatibility of the components. To overcome this problem, several different polymers have been proposed for use as compatibilizing agents in blends of this type as described, for example, in U.S. Pat. Nos. 4,891,405, 4,931,502, 4,912,144, and European Pat. Pub. No. 359,565. While molded products prepared from these compatibilizer-containing compositions exhibit excellent heat resistance, flexural modulus, impact strength, solvent resistance, and resistance to delamination, the compositions are not ideal since the compatibilizer may be either commercially unavailable or overly expensive. It would thus be desirable if the same beneficial improvements in the properties of polybutylene terephthalate could be achieved without the need to use a compatibilizer. Moreover, the incorporation of certain of these compatibilizers into blends of polybutylene terephthalate and styrene/maleic anhydride copolymer has a tendency to lower the heat resistance of the blend slightly as compared to the heat resistance of the blend in the absence of the compatibilizer.
We have now found that thermoplastic molding compositions having superior solvent resistance, stiffness, heat resistance,and processability may be obtained by blending polybutylene terephthalate with a copolymer of methacrylic acid and a vinyl aromatic monomer such as styrene. Surprisingly, the improvement in heat resistance is significantly greater than one would expect from the properties of the unblended components. In addition, the excellent overall balance of properties is achieved without any added compatibilizer, in contrast to the behavior of blends of polybutylene terephthalate with styrene/maleic anhydride copolymers. Without wishing to be bound by theory, it is believed that the acid functionality of the methacrylic acid comonomer interacts through hydrogen bonding with the hydroxyl end-groups, and possibly also with the ester groups of the polybutylene terephthalate. Good interfacial adhesion is thus accomplished without the need for a compatibilizer such as is needed for blends of PBT and styrene/maleic anhydride copolymers.
These results were highly unexpected in view of prior art teachings regarding blends of styrene/unsaturated carboxylic acid copolymers and polyethylene terephthalate, a polyester structurally similar to polybutylene terephthalate except for the presence of oxyethylene units instead of oxybutylene units in the polymer chain. U.S. Pat. No. 4,397,986, for example, teaches blends of polyethylene terephthalate and styrene/methacrylic acid copolymers. The improvement in the heat distortion resistance of a polyethylene terephthalate resin reported is simply that which would be expected in theory from blending a polymer having a high heat resistance (styrene/methacrylic acid) with a polymer having a low glass transition temperature (polyethylene terephthalate). That is, the styrene/methacrylic acid copolymer is dispersed in a continuous polyester phase and behaves in the same manner as a filler. As a consequence of this morphology, molded products made from these blends have poor solvent resistance and tend to delaminate readily.
Japanese Kokai No. 85-149654 teaches blends of polyethylene terephthalate and styrene/methacrylic acid copolymers wherein the amount of copolymer is from 0.1 to 50 parts by weight per 100 parts by weight of the polyester. This publication further teaches that the proportion of copolymer should not exceed 50 parts by weight per 100 parts by weight polyester (i.e., 33 weight % of the total blend) since mechanical properties such as tensile strength will be adversely affected.
Japanese Kokai No. 85-226546 teaches blend compositions comprised of thermoplastic polyester and aromatic vinyl copolymer containing methacrylic acid as comonomer wherein the amount of copolymer is from 2 to 50 parts by weight per 100 parts by weight PET. When the proportion of styrenic copolymer exceeds 50 parts (i.e., 33% of the total blend), the publication further teaches that the thermal resistance will be significantly lowered.
In contrast, the blends of the present invention wherein greater than 35 weight % polybutylene terephthalate is employed as the polyester component do not exhibit any significant loss of tensile strength. Moreover, the heat distortion resistance of the blends of this invention is markedly improved to a degree even greater than expected. Additionally, the instant blends have significantly better resistance towards organic solvents than analogous compositions containing polyethylene terephthalate as the polyester component.